diff --git a/dev/demux.html b/dev/demux.html
index c9c5fd5..04d1e43 100644
--- a/dev/demux.html
+++ b/dev/demux.html
@@ -8,15 +8,6 @@
 	document.body.append(fileInput);
 
 	fileInput.addEventListener('change', async () => {
-		const videoUrl = "https://upload.wikimedia.org/wikipedia/commons/5/53/1941._%D0%9A%D0%BE%D0%BD%D1%91%D0%BA-%D0%B3%D0%BE%D1%80%D0%B1%D1%83%D0%BD%D0%BE%D0%BA.webm"
-		const source = new Mediabunny.UrlSource(videoUrl)
-		const input = new Mediabunny.Input({ formats: Mediabunny.ALL_FORMATS, source });
-
-		const videoTrack = await input.getPrimaryVideoTrack();
-
-		console.log(videoTrack);
-
-		/*
 		const file = fileInput.files[0];
 		const source = new Mediabunny.BlobSource(file);
 
@@ -25,6 +16,15 @@
 			source
 		});
 
+		const videoTrack = await input.getPrimaryVideoTrack();
+		const sink = new Mediabunny.EncodedPacketSink(videoTrack);
+
+		for await (const packet of sink.packets(undefined, undefined, { verifyKeyPackets: false })) {
+			console.log(packet);
+			if (packet.timestamp >= 2.4) break;
+		}
+
+		/*
 		const audioTrack = await input.getPrimaryAudioTrack();
 		const sink = new Mediabunny.EncodedPacketSink(audioTrack);
 
diff --git a/src/isobmff/isobmff-demuxer.ts b/src/isobmff/isobmff-demuxer.ts
index 20bcf42..80c1dc2 100644
--- a/src/isobmff/isobmff-demuxer.ts
+++ b/src/isobmff/isobmff-demuxer.ts
@@ -1459,6 +1459,12 @@ export class IsobmffDemuxer extends Demuxer {
 					const sampleIndex = this.metadataReader.readU32() - 1; // Convert to 0-indexed
 					track.sampleTable.keySampleIndices.push(sampleIndex);
 				}
+
+				if (track.sampleTable.keySampleIndices[0] !== 0) {
+					// Some files don't mark the first sample a key sample, which is basically almost always incorrect.
+					// Here, we correct for that mistake:
+					track.sampleTable.keySampleIndices.unshift(0);
+				}
 			}; break;
 
 			case 'stsc': {